In medium saucepan over high heat, combine the vinegar, salt, sugar, the crushed garlic clove and 1 serrano pepper. Bring to a boil, then turn the heat off and allow to cool.
Put the cucumbers and red onions in a large, nonmetallic serving bowl with remaining serrano pepper and the thinly sliced garlic and toss to combine.
When the vinegar mixture has cooled, strain it into a small bowl and pour it over the vegetables. Stir to coat the vegetables with the vinegar mixture. Cover and refrigerate for at least 1 hour before serving.